The Milky Way Radial Metallicity Gradient as an Equilibrium Phenomenon: Why Old Stars Are Metal Rich
Metallicities of both gas and stars decline toward large radii in spiral galaxies, a trend known as the radial metallicity gradient. We quantify the evolution of the metallicity gradient in the Milky Way as traced by APOGEE red giants with age estimates from machine learning algorithms. Stars up to...
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2025-01-01
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| Series: | The Astrophysical Journal |
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| Online Access: | https://doi.org/10.3847/1538-4357/addbe5 |
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| author | James W. Johnson David H. Weinberg Guillermo A. Blanc Ana Bonaca Gwen C. Rudie Yuxi (Lucy) Lu Bronwyn Reichardt Chu Emily J. Griffith Tawny Sit Jennifer A. Johnson Liam O. Dubay Miqaela K. Weller Daniel A. Boyea Jonathan C. Bird |
| author_facet | James W. Johnson David H. Weinberg Guillermo A. Blanc Ana Bonaca Gwen C. Rudie Yuxi (Lucy) Lu Bronwyn Reichardt Chu Emily J. Griffith Tawny Sit Jennifer A. Johnson Liam O. Dubay Miqaela K. Weller Daniel A. Boyea Jonathan C. Bird |
| author_sort | James W. Johnson |
| collection | DOAJ |
| description | Metallicities of both gas and stars decline toward large radii in spiral galaxies, a trend known as the radial metallicity gradient. We quantify the evolution of the metallicity gradient in the Milky Way as traced by APOGEE red giants with age estimates from machine learning algorithms. Stars up to ages of ∼9 Gyr follow a similar relation between metallicity and Galactocentric radius. This constancy challenges current models of Galactic chemical evolution, which typically predict lower metallicities for older stellar populations. Our results favor an equilibrium scenario , in which the gas-phase gradient reaches a nearly constant normalization early in the disk lifetime. Using a fiducial choice of parameters, we demonstrate that one possible origin of this behavior is an outflow that more readily ejects gas from the interstellar medium (ISM) with increasing Galactocentric radius. A direct effect of the outflow is that baryons do not remain in the ISM for long, which causes the ratio of star formation to accretion, ${\dot{{\rm{\Sigma }}}}_{\star }/{\dot{{\rm{\Sigma }}}}_{\,\rm{in}\,}$ , to quickly become constant. This ratio is closely related to the local equilibrium metallicity, since its numerator and denominator set the rates of metal production by stars and hydrogen gained through accretion, respectively. Building in a merger event results in a perturbation that evolves back toward the equilibrium state on ∼Gyr timescales. Under the equilibrium scenario, the radial metallicity gradient is not a consequence of the inside-out growth of the disk but instead reflects a trend of declining ${\dot{{\rm{\Sigma }}}}_{\star }/{\dot{{\rm{\Sigma }}}}_{\,\rm{in}\,}$ with increasing Galactocentric radius. |
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| institution | Kabale University |
| issn | 1538-4357 |
| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | The Astrophysical Journal |
| spelling | doaj-art-a84f1bbd22344edaa14f431019441bd22025-08-20T03:28:47ZengIOP PublishingThe Astrophysical Journal1538-43572025-01-019881810.3847/1538-4357/addbe5The Milky Way Radial Metallicity Gradient as an Equilibrium Phenomenon: Why Old Stars Are Metal RichJames W. Johnson0https://orcid.org/0000-0002-6534-8783David H. Weinberg1https://orcid.org/0000-0001-7775-7261Guillermo A. Blanc2https://orcid.org/0000-0003-4218-3944Ana Bonaca3https://orcid.org/0000-0002-7846-9787Gwen C. Rudie4https://orcid.org/0000-0002-8459-5413Yuxi (Lucy) Lu5https://orcid.org/0000-0003-4769-3273Bronwyn Reichardt Chu6https://orcid.org/0000-0002-7187-8561Emily J. Griffith7https://orcid.org/0000-0001-9345-9977Tawny Sit8https://orcid.org/0000-0001-8208-9755Jennifer A. Johnson9https://orcid.org/0000-0001-7258-1834Liam O. Dubay10https://orcid.org/0000-0003-3781-0747Miqaela K. Weller11https://orcid.org/0000-0003-4912-5157Daniel A. Boyea12https://orcid.org/0009-0008-8903-160XJonathan C. Bird13https://orcid.org/0000-0001-5838-5212Carnegie Science Observatories , 813 Santa Barbara Street, Pasadena, CA 91101, USA ; jjohnson10@carnegiescience.edu; Department of Astronomy, The Ohio State University , 140 W. 18th Avenue, Columbus, OH 43210, USA; Center for Cosmology & Astroparticle Physics (CCAPP), The Ohio State University , 191 W. Woodruff Avenue, Columbus, OH 43210, USADepartment of Astronomy, The Ohio State University , 140 W. 18th Avenue, Columbus, OH 43210, USA; Center for Cosmology & Astroparticle Physics (CCAPP), The Ohio State University , 191 W. Woodruff Avenue, Columbus, OH 43210, USACarnegie Science Observatories , 813 Santa Barbara Street, Pasadena, CA 91101, USA ; jjohnson10@carnegiescience.edu; Departamento de Astronomía, Universidad de Chile , Camino El Observatorio 1515, Santiago, ChileCarnegie Science Observatories , 813 Santa Barbara Street, Pasadena, CA 91101, USA ; jjohnson10@carnegiescience.eduCarnegie Science Observatories , 813 Santa Barbara Street, Pasadena, CA 91101, USA ; jjohnson10@carnegiescience.eduDepartment of Astronomy, The Ohio State University , 140 W. 18th Avenue, Columbus, OH 43210, USA; American Museum of Natural History , 200 Central Park West, New York, NY 10024, USACentre for Extragalactic Astronomy, Department of Physics, Durham University , South Road, Durham DH1 3LE, UK; Institute for Computational Cosmology, Department of Physics, Durham University , South Road, Durham DH1 3LE, UKCenter for Astrophysics & Space Astronomy, Department of Astrophysical and Planetary Sciences, University of Colorado , 389 UCB, Boulder, CO 80309, USADepartment of Astronomy, The Ohio State University , 140 W. 18th Avenue, Columbus, OH 43210, USA; Center for Cosmology & Astroparticle Physics (CCAPP), The Ohio State University , 191 W. Woodruff Avenue, Columbus, OH 43210, USADepartment of Astronomy, The Ohio State University , 140 W. 18th Avenue, Columbus, OH 43210, USA; Center for Cosmology & Astroparticle Physics (CCAPP), The Ohio State University , 191 W. Woodruff Avenue, Columbus, OH 43210, USADepartment of Astronomy, The Ohio State University , 140 W. 18th Avenue, Columbus, OH 43210, USA; Center for Cosmology & Astroparticle Physics (CCAPP), The Ohio State University , 191 W. Woodruff Avenue, Columbus, OH 43210, USADepartment of Astronomy, The Ohio State University , 140 W. 18th Avenue, Columbus, OH 43210, USA; Center for Cosmology & Astroparticle Physics (CCAPP), The Ohio State University , 191 W. Woodruff Avenue, Columbus, OH 43210, USADepartment of Physics & Astronomy, University of Victoria , 3800 Finnerty Road, Victoria, BC V8P 5C2, CanadaDepartment of Physics & Astronomy, Vanderbilt University , 2301 Vanderbilt Place, Nashville, TN 37235, USAMetallicities of both gas and stars decline toward large radii in spiral galaxies, a trend known as the radial metallicity gradient. We quantify the evolution of the metallicity gradient in the Milky Way as traced by APOGEE red giants with age estimates from machine learning algorithms. Stars up to ages of ∼9 Gyr follow a similar relation between metallicity and Galactocentric radius. This constancy challenges current models of Galactic chemical evolution, which typically predict lower metallicities for older stellar populations. Our results favor an equilibrium scenario , in which the gas-phase gradient reaches a nearly constant normalization early in the disk lifetime. Using a fiducial choice of parameters, we demonstrate that one possible origin of this behavior is an outflow that more readily ejects gas from the interstellar medium (ISM) with increasing Galactocentric radius. A direct effect of the outflow is that baryons do not remain in the ISM for long, which causes the ratio of star formation to accretion, ${\dot{{\rm{\Sigma }}}}_{\star }/{\dot{{\rm{\Sigma }}}}_{\,\rm{in}\,}$ , to quickly become constant. This ratio is closely related to the local equilibrium metallicity, since its numerator and denominator set the rates of metal production by stars and hydrogen gained through accretion, respectively. Building in a merger event results in a perturbation that evolves back toward the equilibrium state on ∼Gyr timescales. Under the equilibrium scenario, the radial metallicity gradient is not a consequence of the inside-out growth of the disk but instead reflects a trend of declining ${\dot{{\rm{\Sigma }}}}_{\star }/{\dot{{\rm{\Sigma }}}}_{\,\rm{in}\,}$ with increasing Galactocentric radius.https://doi.org/10.3847/1538-4357/addbe5Galaxy chemical evolutionMilky Way diskMilky Way evolutionChemical enrichmentChemical abundancesGalactic winds |
| spellingShingle | James W. Johnson David H. Weinberg Guillermo A. Blanc Ana Bonaca Gwen C. Rudie Yuxi (Lucy) Lu Bronwyn Reichardt Chu Emily J. Griffith Tawny Sit Jennifer A. Johnson Liam O. Dubay Miqaela K. Weller Daniel A. Boyea Jonathan C. Bird The Milky Way Radial Metallicity Gradient as an Equilibrium Phenomenon: Why Old Stars Are Metal Rich The Astrophysical Journal Galaxy chemical evolution Milky Way disk Milky Way evolution Chemical enrichment Chemical abundances Galactic winds |
| title | The Milky Way Radial Metallicity Gradient as an Equilibrium Phenomenon: Why Old Stars Are Metal Rich |
| title_full | The Milky Way Radial Metallicity Gradient as an Equilibrium Phenomenon: Why Old Stars Are Metal Rich |
| title_fullStr | The Milky Way Radial Metallicity Gradient as an Equilibrium Phenomenon: Why Old Stars Are Metal Rich |
| title_full_unstemmed | The Milky Way Radial Metallicity Gradient as an Equilibrium Phenomenon: Why Old Stars Are Metal Rich |
| title_short | The Milky Way Radial Metallicity Gradient as an Equilibrium Phenomenon: Why Old Stars Are Metal Rich |
| title_sort | milky way radial metallicity gradient as an equilibrium phenomenon why old stars are metal rich |
| topic | Galaxy chemical evolution Milky Way disk Milky Way evolution Chemical enrichment Chemical abundances Galactic winds |
| url | https://doi.org/10.3847/1538-4357/addbe5 |
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